1. Field of the Invention
The present invention relates generally to heat-shrinkable films suitable for cook-in applications. The present invention is also directed to articles of manufacture which are useful for packaging various products. The present invention is particularly related to a process for packaging a meat product in a heat-shrinkable film.
2. Background of the Invention
Many food products are processed in thermoplastic film packages by subjecting the packaged product to elevated temperatures produced by, for example, immersion in hot water or exposure to steam. Such thermal processing often is referred to as cook-in, and films used in such processes are known as cook-in films.
A food product that is packaged and processed in this manner can be refrigerated, shipped, and stored until the food product is to be consumed or further processed by, for example, slicing and repackaging into smaller portions for retail display. Alternatively, the processed food can be removed immediately from the cook-in package for consumption or further processing (e.g., sliced and repackaged).
A cook-in film must be capable of withstanding exposure to rather severe temperature conditions for extended periods of time while not compromising its ability to contain the food product. Cook-in processes typically involve a long cook cycle. Submersion in hot (i.e., about 55.degree. C. to 65.degree. C.) water for up to about 4 hours is common; submersion in 70.degree. to 100.degree. C. water or exposure to steam for up to 12 hours is not uncommon, although most cook-in procedures normally do not involve temperatures in excess of about 90.degree. C.
It is important that a cook-in film have good inter-ply adhesion and not experience delamination either before, during or after the cooking process. If the inter-ply bond strength of the cook-in film is weak, the cooking process could exacerbate the weak bond strength and cause delamination in the film. Furthermore, if the film has weak inter-ply bond strength, the process of stripping the film from the cooked food product could result in the film delaminating at the weakest link, thereby leaving behind residual film on the cooked product. This is hazardous from a health perspective as cook-in films are often clear, and the customer could inadvertently consume plastic along with the food-product.
Following the cook-in process, the film or package preferably conforms, if not completely then at least substantially, to the shape of the contained food product. Often, this is achieved by allowing the film to heat shrink under cook-in conditions so as to form a tightly fitting package. Alternatively, the cook-in film package can be caused to shrink around the contained food product prior to initiating the cook-in procedure by, for example, placing the package in a heated environment prior to cooking. Also, during cook-in the film should preferably have food product adherence to restrict "cook-out," i.e., the collection of juices between the surface of the contained food product and the food-contact surface of the packaging material. In this manner, product yield is increased by the food product retaining moisture.
Various meat products, such as pork, sausage, poultry, mortadella, bologna, beef, braunsweiger, etc. are prepared as cook-in products. Other non-meat products such as soybean also are considered to be proteinaceous. In all the above cases, it is important for the film to possess good inter-ply bond strength and also important to obtain adequate film-to-food adhesion and provide a snug package for superior aesthetic appearance.
For cook-in applications, packaging materials typically are produced in roll form and then converted into shirred sticks, bags, pouches, etc., for the end user. In the past, heat-shrinkable packaging films which have been provided to the cook-in end-user undesirably have been characterized by inconsistent widths. This inconsistency arises because of two primary reasons. First, heat-shrinkable films have significant free shrink at temperatures as low as 50.degree. C., in some cases 45.degree. C., and in some cases, as low as 40.degree. C. Thus, upon exposure to environments where the temperature exceeds 40.degree. C. and sometimes 45.degree. C., the heat-shrinkable film partially shrinks, causing a change in its width. Obviously, this is more of a problem during storage or transportation of heat-shrinkable films in the hotter summer months. Second, heat-shrinkable films are produced in roll-form. When films are wound into rolls, the leading edge of the film being rolled typically experiences higher tension than the outer portions of the roll. Since polymers are viscoelastic in nature, the film material at the outer portions of a roll (which is under little/no tension) tends to `snap back` like a rubber band, relaxing some of the strain imparted by the orientation process. However, film material at the inner portion of the roll is under tension and therefore unable to relax or snap back. This can result in film which is the first used from the roll, i.e., the outer portions of the roll, having a significantly lower width than film at the interior of the roll. Consequently, heat-shrinkable films can vary in width as much as 5%, and in extreme situations, as much as 7% or 10%.
The marketplace continues to express a need for films which are more consistent in width and are dimensionally stable when exposed to temperatures of 40.degree. C., 43.degree. C., 47.degree. C. or even 50.degree. C. This demand arises from the need to stuff the same quantity of meat product into each casing length, especially where the film is formed into shirred casings. In such cases, the width of the packaging material preferably varies less than 3%, more preferably, less than 2%. Packages produced from shirred casings often are cooked in molds, with the cooked meat-product then being sliced. An inconsistent film width will result in an inconsistent package size and, therefore, in significantly greater yield loss.
To provide end-user with film that is more consistent in width, it is advantageous to anneal the heat-shrinkable cook-in films. Typically, a moving web of film is heated to an elevated temperature in a continuous process. Upon heating the film to an elevated temperature (typically as high as 60.degree. C.), the film shrinks and reduces in width. As a result of this shrinking, the film has now lost some or all of its ability to shrink at or near that elevated temperature. If the process of heat-treatment at an elevated temperature is conducted with a moving web, rather than a roll of film, the process also reduces or eliminates width variation in the film due to the viscoelastic nature of the film (as described above). Thus, the process of annealing produces a film with a more consistent width.
However, the process of annealing as described above can produce certain disadvantages. For example, the annealing process can exacerbate weak inter-ply bond strength in a film. In other words, annealed films can exhibit inter-ply bond strength significantly inferior to that of non-annealed films having the same or similar composition. Often this is characterized by a significantly increased level of tubing striations, wrinkles and creases (i.e., the annealed tubing can have an appearance which is significantly worse than the precursor, unannealed tubing). It is believed that these defects occur in tubing with inherent weak inter-ply bond strength due to differential shrinking of adjacent film layers.
Additionally, cooked packages produced from some annealed films can be relatively flaccid because the film does not fit snugly around the packaged product, creating an unsightly package which includes cook-out from the underlying food product. Such cook-out is evidenced even when the heat-shrinkable film includes a food-contact surface which would be expected to provide adequate film-to-meat adhesion to prevent substantial purge or cook-out.
Thus, providing a film which exhibits good width stability during transportation and storage in hot environments, yet remains suitable for cook-in applications which require good inter-ply bond strength and adequate adhesion remains desirable. Such a film also preferably would have the ability to conform to the food product during cooking and after the cook-in process to prevent substantial purge loss/cook-out, thereby providing a tight, more aesthetically pleasing package.